Thin films may be grown on the surface of substrates by several different methods. These methods include vacuum evaporation deposition, molecular beam epitaxy (MBE), different variants of chemical vapor deposition (CVD) (including low-pressure and organometallic CVD and plasma-enhanced CVD), and atomic layer epitaxy (ALE), which has been more recently referred to as atomic layer deposition (ALD) for the deposition of a variety of materials.
In ALD, the sequential introduction of precursor species (e.g., a first precursor and a second precursor) to a substrate, which is located within a reaction chamber is generally employed. Typically, one of the initial steps of ALD is the adsorption of the first precursor on the active sites of the substrate. Conditions are such that no more than a monolayer forms so that the process is self-terminating or saturative. For example, the first precursor can include ligands that remain on the adsorbed species, which prevents further adsorption. Accordingly, deposition temperatures are maintained above the precursor condensation temperatures and below the precursor thermal decomposition temperatures. This initial step of adsorption is typically followed by a first removal (e.g., purging) stage, where the excess first precursor and possible reaction byproducts are removed from the reaction chamber. The second precursor is then introduced into the reaction chamber. The first and second precursor typically tend to react with each other. As such, the adsorbed monolayer of the first precursor reacts instantly with the introduced second precursor, thereby producing the desired thin film. This reaction terminates once the adsorbed first precursor has been consumed. The excess of second precursor and possible reaction byproducts are then removed, e.g., by a second purge stage. The cycle can be repeated to grow the film to a desired thickness. Cycles can also be more complex. For example, the cycles can include three or more reactant pulses separated by purge and/or evacuation steps.
Ideally, in ALD, the reactor chamber design should not play any role in the composition, uniformity or properties of the film grown on the substrate because the reaction is surface specific and self-saturating. However, only a few precursors exhibit such ideal or near ideal behavior. Factors that may hinder this idealized growth mode can include: time-dependent adsorption-desorption phenomena; blocking of the primary reaction through by-products of the primary reaction (e.g., as the by-products are moved in the direction of the flow, reduced growth rate down-stream and subsequent non-uniformity may result, such as when corrosive and less volatile halide products are produced as a byproduct of an ALD process alternating, e.g., TiCl4+NH3 to produce TiN); total consumption (i.e., destruction) of the second precursor in the upstream-part of the reactor chamber (e.g., decomposition of the ozone in the hot zone); and uneven adsorption/desorption of the first precursor caused by uneven flow conditions in the reaction chamber.
These problems have been partially alleviated with the use of a showerhead-type apparatus used to disperse the gases into the reaction space, such as disclosed in U.S. Pat. No. 4,798,165. The showerhead-type apparatus, as found in U.S. Pat. No. 4,798,165, may be positioned above a substrate so that the reactants and purge gases flow through apertures that are located on the showerhead and the gas flow may be directed perpendicular to the substrate. However, in such a configuration, in the course of time the reacted gases may form a film in the apertures and the apertures may become blocked. Such blockage may result in uneven deposit of layers onto the substrate.
PCT publication No. WO 00/79019, published Dec. 28, 2000 discloses use of hollow tubes with apertures for ALD deposition. In addition to issues with respect to blockage of the apertures, the disclosed structure contemplates relative rotation of either the substrate or the tubes during deposition. Such a construction leads to the additional issue that, for most efficient saturation of the substrate with reactant, rotation must be calculated to be an integral value in each reactant pulse, limiting flexibility in recipe design and risking non-uniformity. Furthermore, the complexity of rotating elements leads to risks of reactant leakage between rotating parts, consequent particle generation and/or safety hazards.
Thus, there is a need for an improved apparatus and method for depositing thin layers that addresses at least some of the problems described above.